The instant invention relates to a screw-type clamp suitable for lifting material such as metal plates.
The prior art contains numerous types of clamps suitable for lifting metal plates. The majority of these prior art clamps incorporate one generally fixed jaw and another movable jaw which may be brought into contact with the material to be lifted. The movable jaw has generally been activated by means of a lever or similar structure.
A few of the prior art devices have incorporated a threaded shaft for advancing one jaw into engagement with the material to be clamped, as illustrated in Sherwood U.S. Pat. No. 2,548,401 and Gardner U.S. Pat. No. 3,269,766. However, these prior art devices have suffered several significant disadvantages in operation. Devices such as disclosed in the Sherwood patent, having one rigidly fixed jaw and a second jaw capable only of threaded axial movement, have suffered from an inability to be tightened sufficiently against the material to be carried, so that such clamps are usable only for lifting relatively light loads. Devices such as disclosed in the Gardner patent have overcome some of these lifting limitations by incorporating one jaw which is capable of limited translational movement vertically within the slot and a second jaw which is attached by a ball and socket joint to the end of the clamping screw and which includes a toothed, convexly curved face. However, when a load is applied to the Gardner structure, as during lifting, the rotation of the convexly faced jaw about its ball and socket mounting to grip the lifted plate more securely causes the point of application of the force between the plate being lifted and the screw to move away from the axis of the screw, thus creating a bending moment on the screw. Due apparently to the imposition of this bending moment, it has been found in practice that the screw tends to bind within its threads thus making subsequent release of the clamp very difficult when significant lifting loads have been applied.